The present invention relates to a processing method of a transfer optical surface to transfer an optical surface onto an optical element, a processing device, and an optical element molding use die, and particularly to a processing method of the transfer optical surface appropriate for a cutting processing of a small optical surface, a processing device, an optical element molding die by which such an optical surface can be formed, and a diamond tool.
In order to mold or produce an optical element by using an optical material such as a glass or plastic, when the optical material which is heated and softened, is injected into the cavity formed by the molding die (hereinafter, also called molding die) for the optical element, with a high pressure, or when it is pressed by a molding die (a pressingly forming die) and cooled and solidified, the transfer optical surface shape or surface roughness owned by the die, is molded and transferred to the optical material, and the optical surface is formed, the optical element can be efficiently produced. Because the molding die having this transfer optical surface is brought into contact with the heated optical material with the high pressure, the conditions to secure the long working life of the molding die, such as a sufficient heat resistance, a low reactivity or wettability so that the optical material is not adhered, and a high and not easily flawed hardness are necessary, and simultaneously, a condition that the processing is easy and the productivity is high, is important to reduce the cost of the molding die production, and as a result, to lower the cost of the optical element, and realize an efficient productivity.
Conventionally, in order to produce the molding die for molding the optical element whose material is glass, material whose heat resistivity is high, such as the ceramic or cemented carbide material is attached to the main shaft of the processing device as shown in FIG. 8, and it is processed into the transfer optical surface shape by the cutting processing by a grinding stone using diamond abrasive grains, and in order to further improve its surface roughness, the grinding is conducted as the after processing. Then, when, on the surface of the processed transfer optical surface, the protective coating is conducted by the material such as carbon whose wettability with the glass is small, boron nitride of the thickness of about 100 nm to 1 μm to prevent the adhesion to the glass, or precious metal, the molding die is produced. Relating to the production method of the transfer optical surface of these molding dies, is generally described in the patent references 1-5. As the ceramic material used for the molding die, the molding die described in Table 1, and it is seen that the hardness is almost not smaller than the Rockwell hardness HRA 90 and is very hard material.    [Patent reference 1]
Tokkaisho-No. 52-45613    [Patent reference 2]
Tokkohei No. 2-1782    [Patent reference 3]
Tokkohei No. 3-55421    [patent reference 4]
Tokkohei No. 3-59016    [Patent reference 5]
Tokkohei No. 3-61617    [Not-Patent reference 1]
“The study of ductile mode cutting processing of the optical glass by single point diamond machining” by Ichiro Ogura and Yuichi Okazaki in the precision engineering society magazine Vol. 66, No. 9, 2000.
TABLE 1FractureCriticaltoughnessYoung'soverbearName of ceramicvaluemodulusdepthHardnessmaterial(MN/m3/2)(Gpa)dc (μm)(HRA/HV)Alminum nitride3.03102.79  89/1000(AIN)Zirconia (ZrO2)10.021010.891.2/1250Silicon7.53102.7593.5/1850nitride (Si3N4)Silicon3.24200.31  94/2400carbide (SiC)Silicon490  97/2800carbide (CVD-SiC)Alumina (Al2O3)4.24021.0393.6/1900Boronic carbide97.5(B4C)Chrome carbide4.03723.44  91/1200(Cr3C2)Titanium50094  boride (TiB2)
Relating to the molding die using the ceramic material, in the case where its point of the problem is further detailed, when the powder of the row material is sintered, because even when it is the normal pressure sintering or the high pressure sintering, the gap between its powder remains also after sintering, and it becomes a residual bore, there is a case where a minute hole is generated on the optical surface when the cutting processing or grind processing is conducted. This minute hole becomes a cause that it makes the useless minute convex portion, that is, a defect generate on its optical surface when the optical element is molded, or the glass material is fused to the molding die. Therefore, normally, the following processing is conducted that the rough shape processing of the optical surface is conducted by the cutting processing or in the case of conductive ceramic, by the discharge processing described in Tokugan No. 2002-017122, Tokugan No. 2001-359838, on the sintered mold material, and the CVD (Chemical Vapor Deposition) coating of the ceramic material whose linear expansion coefficient is about the same, is conducted from the film thickness 10 μm to 5 mm, and minute ceramic layer is provided, and the cutting processing or grind processing is conducted on this ceramic layer, and the optical surface shape such as an aspheric surface is accurately produced. That is, in the die for optical element molding by the mold material of the ceramic, the main body is the powder sinter material, however, it is common that the transfer optical surface portion is made a minute ceramic by the CVD coating, and in order to generate the transfer optical surface shape on such a very minute and high hardness ceramic material at the shape accuracy 50 nm or not higher than that, the cutting processing is conducted by the grind stone using the diamond abrasive grain, and further, the grind processing is conducted as the after processing for the improvement of its processing surface roughness.
Then, in order to enhance the mold releasing property from the glass, on its optical surface, the film of DLC (Diamond Like Carbon) or boron nitride which has a poor wettability to the glass is formed in the thickness not larger than 1 μm by the ion plating or spattering, and it is used for the molding.
As an index showing the processing difficulty of the ceramic material, the value in which the critical pressing depth dc of the transition area under which the brittle fracture is caused, is calculated from the ductility condition when the diamond pressing piece is pressed in the ceramic material, shown in Table 1. This is the value calculated by the following expression (1), and the limitation value at which the material is brittle-fractured when the pressing piece is pressed-in more than this value.
[Arith. 1]
                              d          ⁢                                          ⁢          c                =                              E                          H              ⁢                                                          ⁢              v                                ⁢                                    (                              K                                  H                  ⁢                                                                          ⁢                  v                                            )                        2                                              (        1        )            Where, dc: critical overbear depth (μm), E: Young modulus (GPa), Hν: Vickers harness (GPa), K: fracture toughness value (MPam1/2)
When actually a grind stone or a cutting edge of the cutting tool is cut in the material, it is said that when it is not a fraction of the critical overbear amount, because the processing is advanced by the brittle fracture, the optical mirror surface can not be attained. That is, even in the case of zirconia whose critical overbear amount is maximum, when the depth of cut of the cutting edge of μ-order is not stably realized, the mirror surface can not be generated, and in the above patent references, in the case of silicon carbide which is widely cited as the material of the die for the optical element molding, it can be seen that the transfer optical surface can not be generated when the depth of cut is not at least about 100 nm. As described above, when the transfer optical surface is generated and processed on these ceramic material, it can be said that the processing is very low efficient and the processing of the high difficulty. Because of such a reason, by the cutting tool such as the diamond, by the minute depth of cut of the only one cutting edge, it can be said that a fact that the high accurate transfer optical surface is generated and processed on these ceramic material, is inefficient and the tool abrasion is conspicuous, and it can be said that conventionally, it is unthinkable. Further, such instances are not believed to exist. Accordingly, conventionally, it is general that, by using the diamond grind stone, by the cutting processing by which the processing efficiency can be maintained high by cutting edges of a plurality of abrasive grains, the transfer optical surface shape is generated, and after that, it is ordinary that a scratch generated by the cutting processing or a chatter mark is removed by the grind processing, and a smooth optical surface is generated.
Further, when the optical element molding die is generated by the cemented carbide material, in the general use grade of the cemented carbide material which can be comparatively easily obtained, there is a material in which cobalt of 10 and several mol % is mixed in grains of tungsten carbide (WC) as the binding material and sintered, however, because cobalt is low in the thermal resistance, and when the temperature is increased high, there is a characteristic that it is easily oxidized, it is not appropriate also for the material of the optical element molding use die by which a very common glass for molding whose glass transition point is from 500 C to 600 C, is molded. Further, when the content of cobalt is large, the specific gravity or hardness of the cemented carbide material is decreased, however, because also the organization structure after the sintering, is thin, when the transfer optical surface is generated by the cutting processing or grind processing, the surface roughness can not be improved. Accordingly, as the material for forming the transfer optical surface, it can be said that a case where the content of cobalt which is the binding material, is small, is better. Further, when the particle diameter of tungsten carbide (WC) is decreased, because the organization structure after the sintering, is fine, the minute hole (residual bore) when the transfer optical surface is generated, is also decreased, and the surface roughness is improved, however, on the contrary, there is a problem that, because the hardness is increased and the value of fracture toughness is lowered, the brittle fracture is easily caused, and the processing difficulty is increased.
Further, when it is high temperature-sintered by pressing isotropically by argon or nitrogen gas by the HIP processing (hot hydrostatic press)at the time of sintering, because the distance between powders is uniformly compact at the sintering and the density is increased, the residual bores are reduced, and the surface roughness in the transfer optical surface is improved, however, because the hardness is increased corresponding to that, the processing difficulty is further increased. These characteristics of the cemented carbide material are collected in Table 2.
TABLE 2WCFractureCriticalparticleAn amountSinter-toughnessYoungcompressdiameterof cobaltingSpecificvaluemodulusdepth dcHardness(μm)(mol %)methodgravity(MN/m3/2)(GPa)(μm)(HRA/HV)1014*113.081.5108*114.312.654056.488/115015HIP15.0920.5noneHIP14.76.56001.8395/2400*1: Hot press (single shaft press)
As described above, when the composition, particle diameter, or recipe which are appropriate as the transfer optical surface material, are selected so that they are appropriate for the optical element molding use die, the hardness of the cemented carbide material for the transfer optical surface is increased on and on, and finally, it becomes not smaller than Rockwell hardness HRA 90. When trade names of the cemented carbide material actually put on the market are viewed, in the general use cemented carbide material C95 (product name) by Fuji dies (Co.) in which cobalt content is 10 and several %, Rockwell hardness is about HRA 81.5, however, in the cobalt-less cemented carbide material RCC-FN (product name) by Nippon tungsten (Co.) which is particularly processed for the purpose of use of the optical element molding use die, the Rockwell hardness reaches HRA 95 by the increase of fineness and HIP processing, and increase of cobalt-free. Relating to these cemented carbide material, when the critical compression depth which is transited from the ductility area to the brittleness area when the diamond pressing piece is compressed, is calculated, in the same manner as the case of ceramic material, from the fracture toughness value, Young modulus, or hardness, as shown in Table 2, in the general use cemented carbide material in which the cobalt content is slightly decreased, it is 56.4 μm, in contrast to that, in the cemented carbide material for the purpose of use of the optical element molding use die, it is 1.83 μm, and it can be seen that it is decreased to about 1/30. When the cutting depth of the cutting edge at the grinding processing or cutting processing is not made to a fraction of this compression amount, because it is said that the material is brittle fractured, and the optical mirror surface is not formed, and also in the case where the transfer optical surface is generated by the cemented carbide material, it can be understood that, when the material is appropriately selected, in the same manner as the case of ceramic material, it is necessary that, by a very minute cutting amount of 1 μm or less than that, the processing is conducted at a low efficiency.
As described above, because the optical element molding use die produced by the appropriately selected cemented carbide material, is very hard and easily brittle fractured, the processing is difficult, and in the optical element molding use die by these materials having the transfer optical surface shape such as the aspheric surface having actually the curvature, there is no example that the transfer optical surface is accurately generated and processed by the cutting processing by a single cutting edge, and no one has such an idea. Accordingly, conventionally, it is normal that, by using the diamond grind stone, the transfer optical surface generation by the grinding processing by the cutting edge of many abrasive grains, by which the processing efficiency can be maintained high, is generally conducted, and after that, a scratch or chatter mark by the grinding processing is removed by the polishing processing, and a smooth optical surface is generated. Then, on the transfer optical surface, in order to enhance the mold releasing property from the glass, the film of platinum and the alloy of iridium which are poor in the wetness with the glass, is formed into the thickness not larger than 1 μm, and it is used for the molding of glass optical element.
On the one hand, in the die for molding of plastic optical element, the steel material or stainless material refined as the base material is used, and the transfer optical surface shape is cutting processed in the accuracy of about 10 μm by using the general purpose lathe, and is made a blank type. Further, on the transfer optical surface, the electroless nickel plating in which the diamond cutting is possible, is adhered in the thickness of about 100 μm, and by using the hyper precision lathe as shown in FIG. 6, and further, by using the diamond cutting tool as shown in FIG. 7, the mirror surface cutting is conducted, and the transfer optical surface is generated and processed.
However, also in the die for molding of plastic optical element, in the case where its number of production is very large, it reduces the trouble of the die exchange or the stop time of molding that the life of the die is prolonged as long as possible, and becomes important for realizing the good efficient production. Conventionally, the die material such as the above-described electroless nickel plating can easily conduct the diamond cutting, however, on the other side of it, as the hardness, in the Vickers hardness, the upper limit is to about Hv 550, and even when the hardness is increased when the heat processing is conducted and it is crystallized, the upper limit is to about Hv 650 in Vickers hardness. Further, in the molding of plastic optical element, the mold releasing property of resin material and its die material is comparatively good, however, because the working oil is adhered to the transfer optical surface or the resin material is rarely fused, the operation to remove them is necessary, and there are many cases where the die has minute flaws resulting from the wipe-out operation.
Further, in order to take out the molded optical element, in the case where the die parts having the transfer optical surface have the sliding portion by which its transfer optical surface is protruded, the weariness is generated in the die parts by the sliding, and there is a case where the wandering is generated in the engagement portion. Because this engagement portion has the function of the guide by which the situation that the transfer optical surface is accurately set into the molding die set so that the eccentricity is not made in the transfer optical surface, is maintained, the generated wandering causes the slippage or the eccentricity such as tilt or shift of the transfer optical surface, and as the result, there is a possibility that the slippage or eccentricity is transferred onto the optical surface of the molded optical element, and the optical performance of the molded optical element is lowered. As one method for solving this, there is a method in which the conventional steel material, stainless steel material or electroless nickel plating is not used for the die material, but by using the cemented carbide material, there is a case where its transfer optical surface or sliding portion is formed, thereby, the flaw or abrasion can be prevented, and the life of the die can be prolonged. In the case of the die for molding of the plastic optical element for which such a cemented carbide material is used, the generation of its transfer optical surface or sliding portion is, conventionally, in the quite same manner as the case of the die for molding of the glass optical element, the grinding processing by using the diamond grind stone and the polishing processing for improving the processed surface roughness are conducted as after-processing.
Hereupon, in the case of the die for molding of the plastic optical element, because the softening temperature of the plastic material is about 250° C., which is low, and further, it is rare that the resin material is adhered onto the surface of the die of the cemented carbide material, there is the actual condition that to form the film of the mold releasing material on the transfer optical surface is rarely conducted.
From the above description, it can be said that as a large factor by which the processability of the transfer optical surface formed of ceramic material or cemented carbide material is deteriorated, the hardness of these materials are very high and brittle. That is, because the hardness is high, the processing efficiency is very bad, the wear of the tool is conspicuous, and as the result, it is not only difficult that a predetermined processing condition is stably maintained, but, further, because it is brittle material, when the depth of cut of the cutting edge is large, because the processing advances by the fracture, it is necessary that a notching amount is made a very minute depth of cut not larger than 1 μm, and the processing is made advance in the ductility mode. As described above, even when any tool is used, unless the processing is conducted in this ductility mode, the transfer optical surface can not be generated on the material of ceramic or cemented carbide by the machining.
Conventionally, a reason that the generation of the transfer optical surface is conducted by the cutting processing of the ceramic material or cemented carbide material having such a nature, is in the case of cutting processing, even when the diamond which is hardest, is used for the cutting edge, when these materials of high hardness are cutting processed for a long period of time, with an amount of a minute depth of cut by a single cutting edge, because the addendum is quickly worn, and the shape of the addendum is changed, the desired transfer optical surface shape can not accurately be generated. Or, it is due to the thinking that the cutting power is lowered by the quick wearing of the cutting edge and the function as the tool can not be attained. Further, because the cost of the diamond tool is very high, the concerned party easily have the evasion feeling to the cutting processing in which it is thought that the cutting edge is simply worn in this manner and the function is not performed, or in almost references relating to the diamond cutting processing, the content that “the material appropriate for the transfer optical surface generation by the diamond cutting processing is only the soft metal such as copper or aluminum and electroless nickel plating” is written, and due to that a preconception that the diamond cutting processing is not appropriate for the transfer optical surface generation of the hard material such as the cemented carbide or ceramic, is generated, there is an actual situation that it is considered that even the study for putting to practical use is the senselessness.
For example, in the above non-patent reference, the ductility mode processing of silicon carbide (SIC) which is typical ceramic material as the die for the optical element molding of the glass optical element, is described by comparing to BK7 of the optical glass or melting quartz, however, in this conclusion, it is concluded that “BK7 can be easily processed, however, in SIC or melting quartz glass, the crack-free processing is hardly conducted.” Even in such a study, conventionally, there is a general thinking that it is not advantageous that the transfer optical surface is generated by the cutting processing of ceramic material or cemented carbide material.
Further, because, in the cutting tool, the elastic deformation is hardly made in the cutting edge at the time of processing, and a notching amount is almost the same as the removal amount, in order to conduct the ductile mode processing in the cutting processing, the high cost hyper precision processing machine by which the notching amount of about 100 nm can be correctly maintained, is necessary. Moreover, in order to fully perform the accuracy of the processing machine, because it accompanies the increase of the cost of the auxiliary facility such as the temperature control of the installation environment or the removal of the floor vibration, it is introduced to the increase of the cost of the die processing, and finally to that of the molded lens.
In contrast to that, in the grinding processing, it is characterized in that: because the diameter of the grinding stone is large and the processing point is on the circumference of the grinding stone comparing to the cutting processing, the processing is conducted by the cutting edge of numerous abrasive grains, and the processing burden of each cutting edge is small and the wearing is small, and even when it is worn, because the grinding stone is rotated, and it always performs as the tool of the true circle, it hardly has a bad influence upon the processing surface shape, and it is hardly processed into the large different shape by which the aberration of the higher order component is generated in the lens performance, and at the time of processing, when the grinding stone is pressed onto the processed material, because it is elastically deformed and yielded, it does not become a compulsory notching, and even when a notch of micron order is given onto the grinding stone, the cutting edge of the abrasive grain practically has a notch not larger than tenth of it, and the ductility mode processing can be comparatively easily realized, and the processed surface close to the optical mirror surface can be obtained.
From such a reason, in order to generate the transfer optical surface on the ceramic material or cemented carbide material, conventionally, not the cutting processing, but the grinding processing and polishing processing as the after processing of that, are conducted in a practical manner.
Certainly, these characteristics play well when particularly the transfer optical surface of the die for the optical element molding is not smaller than 5 mm in the diameter and comparatively large, and in order to generate the transfer optical surface on the material of the cemented carbide or ceramic, the grinding processing is overpoweringly advantageous. However, recently, for example, in an image pick-up lens of a micro camera installed in a carrying telephone, the central radius of curvature of its aspheric surface optical surface is not larger than about 2 mm and the diameter is very small, and also a deep shape in which the maximum normal line angle of the optical surface is not smaller than 65°, is not rare. This means that, in the case where the transfer optical surface is generated by the cutting processing, because the cutting length is reduced by the reduction of the diameter, the wear of the cutting edge of the tool is not so large, and when an appropriate correction processing is jointly used, the shape accuracy of the transfer optical surface can be generated sufficiently high. Further, when the outer diameter is reduced and the radius of curvature of the transfer optical surface is reduced in proportion to it, particularly, in the case where the shape of the processed surface is the concave surface, because it is necessary that, in order to put the grinding stone in a minute concave surface, it is reduced, and the length of the circumference which is a processing section of the grinding stone, is also reduced, and the burden of the cutting edge is increased, and the cutting ratio is started to be rapidly lowered. That is, the difficult points are generated in which, during the grinding processing, the condition of the cutting edge of the abrasive grain is quickly changed and the reproduction of the processing becomes poor, because the diameter of the grinding stone is reduced soon, it is necessary that its correction is frequently conducted, or in order to increase the efficiency of the grinding and increase the peripheral speed of the grinding stone, the high cost high frequency spindle which is rotated at very high speed, is necessary for the shaft of the grinding stone.
Further, in the concave transfer optical surface whose diameter is small and which is very deep, problems are generated in which, because it is necessary that the diameter of the grinding stone is not only reduced, but also the shaft of the grinding stone is made small, and because the bending rigidity of the shaft of the grinding stone is lowered, and the position of the processing point of the die is unstably fluctuated, or the shaft of the grinding stone is brought into an end surface of the processed material, it is necessary that the optical axis which is the grinding stone shaft and the processing rotational shaft of the die, is tilted by about 45° from the orthogonal arrangement. Particularly, in the latter case, because, when the grinding stone is worn during the processing, the processing point of the grinding stone is moved to the tilted direction of the grinding stone shaft, the processing point on the die is moved, and the processing situation is largely changed and unstable. In the case where the grinding stone shaft and the processing rotational shaft of the die are the orthogonal arrangement, even when the grinding stone is worn, because the processing point is not largely moved and only the grinding stone radius is changed, as described above, the processing shape is not so largely influenced. However, as the tendency of the recent optical element, because the optical element of the small diameter and small radius of curvature is increased, there is an actual situation that such a system of the characteristic of the grinding processing can not be used efficiently.
Further, because the radius of curvature of the transfer optical surface is close to the radius of the grinding stone, when the step difference which is the attaching reference of the optical element molded on the outer peripheral portion of the transfer optical surface is going to be provided, there is a case where, because this portion generates the tool interference during the grinding processing, it can not be provided on the die for molding. Therefore, the reference surface when the optical element is attached, does not become always the best position on which the molding optical element is attached, and even to the shape of a part of the other part to be attached, there is a case where the limitation of the design work is given.
When recent problems of these grinding processing are said in one word, it is the following in which, when the processed surface is reduced, the grinding ratio is rapidly deteriorated, and the processing efficiency is lowered, the reproducibility is reduced in the cutting edge or processing condition by the wearing of the grinding stone, and the high accurate transfer optical surface generation becomes difficult.
As the after processing, in the polishing processing by which the surface roughness of the transfer optical surface is improved, when the polishing processing is conducted so much as to eliminate the scratch like surface flaw generated by the grinding processing, the case where the shape of transfer optical surface generated by the grinding processing with much efforts is destroyed, is generated. Accordingly, it can be said that the polishing processing as the after processing of the grinding processing is also, conventionally, the processing method whose reproducibility is insufficient.